Multilateration is a cooperative surveillance technique used to locate the source of a radio transmission based upon differences in time of arrival (DTOA) and/or Time-of-Arrivals (TOA) of a radio signal received at multiple receivers of known position. Signal arriving time measurements are often referred to as Time-of-Arrival (TOA) measurements and the difference between two TOAs is often referred to as a Difference-Time-of-Arrival (DTOA) measurement. Existing systems for determining locations of radio signal emitters based on signal TOA and DTOA are generally described as multilateration (MLAT) systems.
MLAT systems have been deployed to locate aircraft, for example, using an aircraft-generated signal such as a signal from an Air Traffic Control Radar Beacon System (ATCRBS), a Mode-S transponder system, or an Automatic Dependent Surveillance Broadcast (ADS-B) system. In such systems, a plurality of sensors or remote units (RUs) measure the signal times of arrival (TOA) at carefully sited and surveyed locations to provide the coverage necessary for emitters in a predefined region (e.g., around an airport). Each RU utilizes a clock that is synchronized to a common time base (e.g., global positioning system (GPS) time may be used as the common time base). When a target-generated signal is received at an RU, the received signal is time stamped and forwarded to a central processor or other designated location via a radio frequency (RF) or a hard-wire network, where the received signal and time stamp information gathered from all RUs is used to compute the origin of the transmission based upon the differences in propagation time of the target signal received at multiple RUs. More specifically, the RU-measured TOAs are routed through data links or other communication networks to a centralized processor, where DTOA calculations and position estimations are performed.
In one instance, the measurement of range defines a mathematical sphere on which the transmitter is located, with the receiving RU located at the center of the sphere. The DTOA between a pair of the measurements of signal TOA define a hyperboloid on which the transmitter is located, with the RUs located at the two foci of the hyperboloid. The difference between signal TOA and the signal transmitting time defines the range to the target, because distance and time are related by the speed of light (a constant). However, to obtain direct traveling time measurements, RUs and emitters are required to be synchronized. One method able to avoid the synchronization requirement between emitters and RUs is by calculating the range from the round-trip traveling time minus the expected delays. The round-trip technique requires cooperation from emitters and requires RUs to actively interrogate emitters. When active interrogation is undesired and is not performed, the solutions are based on DTOA measurements. The underlying mathematical problem of the multilateration system is the problem of solving the intersection of the measurement hyperboloids and/or spheres, which is equivalent to the problem of solving the DTOA and/or range equations, given exact RU locations and synchronized RU clocks.
As with any detection and location system, there are errors associated with a multilateration system. For example, each RU will have errors based on RU inherent properties such as clock drift and system processing latency. Prior art methods for reducing the clock drift error have utilized a reference transmitter to time synchronize the RUs in the multilateration system. A reference transmitter is located at a known position that is visible to each RU. Referring to FIG. 1, for example, the reference transponder 12 broadcasts a signal and the actual time of arrival (TOA) of the reference transponder signal is recorded and reported by each fixed surveyed RU 10 (RU1-RU3) to a central processor 11. In addition, since the position of each RU 10 is known and the location of the reference transponder 12 is known, the expected time of arrival (TOA) of the reference transponder signal can be calculated.
The time of transmission from the reference transponder plus the propagation times to the individual RUs may then be compared to the actual TOA at the RUs. For multilateration calculations depending on the differences in time of arrival, correction of the DTOA is sufficient to correct the system. The actual time differences between TOAs are compared to the calculated values and corrections are made to each RU to adjust for the errors in DTOA. Any subsequent signals received are corrected with an individual correction time for each RU thereby calibrating the system of RUs used for multilateration. These corrections are made prior to multilateration calculations. A system using this reference transponder calibration technique is described in U.S. Pat. No. 5,424,746 to Schwab.
One notable limitation for using the known multilateration techniques is that each RU 10 and the reference transponder 12 are required to be physically located at carefully surveyed locations. As described above, to determine emitter locations, multilateration systems require prior knowledge of RU locations. For existing multilateration systems, RU 10 locations are precisely surveyed prior to operation and once the survey is complete the RUs are kept stationary. Relocating or moving an operating RU is unlikely and in any event must be done offline since the uncertainty incurred due to movement of the RU greatly degrades the overall position accuracy of the system. The immobility of the RUs limits such multilateration systems to applications where the RUs are stationary.
In emergency or disaster situations, the stationary RUs that are located at surveyed locations are frequently inoperable due to damage and/or lack of power. For example, government officials are often unable to re-open an airport for several days after a disaster (e.g., a hurricane), because the local surveillance systems are not functioning. The inability to provide sufficient surveillance to re-open an airport in turn creates a bottleneck slowing the flow of relief aid into the affected area. As disaster aid support requirements or terrestrial conditions change, having a multilateration system capable of operating without the need to use pre-surveyed RUs or even while the system's RUs are mobile (or are being reconfigured) would be very beneficial for disaster relief efforts, as well as in other situations.
Another notable limitation on using known multilateration techniques is the need for a reference transponder 12 that is located where the reference transponder has a clear view of each RU to time synchronize or calibrate the multilateration system.
For many advent applications in the fields such as law enforcement, emergency/disaster response, and on-demand or temporary-coverage services, it would be very helpful if unsurveyed RUs could be used, and even better would be a technique where the RUs are allowed to move while operating. In light of the above, there is a need for a system and method for monitoring the position of a target that does not require RUs to be stationary at known, carefully surveyed sites, and that can operate while the RUs are moving.